1. Field of the Invention
The present invention relates generally to an ink-jet printing apparatus and an ink-jet printing method. More specifically, the invention relates to an ink-jet printing apparatus and an ink-jet printing method for performing printing by ejecting an ink and a processing liquid for making coloring agent in the ink insoluable or coagulated, to a medium to be printed.
The present invention is applicable for all of devices or apparatus employing paper, cloth, non-woven fabric, OHP sheet and so forth as a printing medium. In concrete terms, the present invention is applicable for office appliances, such as a printer, a copy machine, a facsimile and so forth, and industrial production machines or so forth.
2. Description of the Prior Art
An ink-jet printing system is a non-impact type printing system for printing characters, graphic images and so forth on a printing medium, such as a paper, cloth, plastic sheet and so forth (hereinafter referred to as a printing paper) by directly ejecting an ink, and thus is a low noise printing system. Also, this system is advantageous for low running cost and ease of down-sizing and providing color printing capability for the apparatus, for the reason that no uncomplicated apparatus is required in implementation. Since the ink-jet printing system holds various advantages stated above, the system has been widely employed in printers, copy machines, facsimiles, wordprocessors and so forth.
On the other hand, in case of a color printing apparatus employing the ink-jet system, it is occasionally required to use a dedicated printing paper for obtaining a highly developed color image without bleeding between different colors of inks. However, in the recent years, according to improvement of the inks, inks adapted to printing on a plain paper have become available for practical use.
However, even in employing such inks, it is still possible that the quality of printing becomes unsatisfactory depending upon the printing condition.
More specifically, when a color image is printed on the plain paper, in consideration of bleeding, a quick dry ink having high permeation speed to the plain paper is employed. However, in such case, while bleeding between the inks can be prevented, the portion printed in black causes lack of density and the portion printed in other color of ink is poor in color development. Also, in case of a line image, typically characters and so forth, blurring, referred to as feathering, along the fiber of the paper becomes perceptible so that quality of a printing image is degraded. The characters of a black ink specifically becomes perceptible and have no sharpness.
On the other hand, in the ink-jet printing system, various measures set out below have been known for enhancing the density of black image to be printed.
First of all, there is a method to eject black inks for a plurality of times for the same pixel. In this method can be realized by so-called multi-path printing system, in which scanning for one pixel is performed for a plurality of times, or by a system lowering a carriage speed without varying the ejection frequency. Both systems inherently cause a derivative problem of lowering of the printing speed to cause lowering of the through-put of the overall apparatus. Also, in such method to eject the ink for a plurality of times, the possibility of bleeding of the ink becomes high. If the bleeding is caused between the black image portion and other color image portion, the image quality can be significantly degraded.
On the other hand, as second and third measures, a method of increasing an ink ejection amount for the black ink or a method of ejecting other color ink on the black ink have been known in the art.
Increasing the ejection amount of the black ink can be realized by widening an area of an ejection heater of an ink-jet head and thus increasing thermal energy to be applied for the ink, for example. However, when the ink ejection amount is increased in this manner, for the reason of increasing of influence of pressure wave of the ink, a refill frequency is generally lowered. Also, when the ejection heater area is increased, it may cause increasing of power consumption.
In the third method to eject other color ink overlapping with the black ink, increasing of density is relatively low despite of increasing of the ejected ink amount. To the contrary, a significant problem of thickening of the character may be caused. A reason why satisfactory gain of density cannot be obtained in this method is that in view of necessity of avoiding bleeding for ejection of large amount of ink to the printing medium, ink having high permeation ability is employed to be quickly permeated in a printing medium to lower the effect of overlapping ejection.
As a fourth method for increasing density of the black image, it has been known to accelerate drying of the ink by means of a heater or so forth. This method is effective for preventing bleeding of ink which may otherwise be caused when a large amount of ink is ejected as set out with respect to the first to third methods and to enhance color development. However, this method inherently causes increasing of a cost of the apparatus and of a size of the apparatus.
Furthermore, as a fifth method for increasing the density of the black image, it has been known to increase density of the dye. However, increasing of dye concentration in the ink inherently causes increasing of a viscosity of the ink to increase the possibility of solidifying of the ink in ejection orifices or ink passages of the ink-jet head to make it difficult to assure reliability of ejection.
In addition, in a different viewpoint from the methods set forth above, namely for directly increasing the density of the printed image, a method for performing printing by ejecting an ink and a processing liquid for making the ink insoluble for prevention of bleeding of ink and fastness of the printed image, has been known in the art.
This method may be realized in two constructions. One is to employ an achromatic processing liquid. Another is to employ a chromatic processing liquid, namely to make the processing liquid serve as one of the inks.
The latter has an advantage in that no additional head for the processing liquid is required. One example has been disclosed in Japanese Patent Application Laid-open No. 105773/1989. Also, the former has been disclosed in Japanese Patent Application Laid-open No. 128862/1983.
However, the above-identified two publications, in which the processing liquid or the processing liquid serving as the ink is employed, there is only disclosed constructions for ejecting ink and so forth from a plurality of heads in an overlapping manner. These publications do not have a positive statement for increasing of the density of the lower image.